1. Field of the Invention
Embodiments of the present invention generally relate to the field of light-emitting diode (LED) technology and, more particularly, to a vertical light-emitting diode (VLED) structure.
2. Description of the Related Art
LEDs have been around for several decades, and research and development efforts are constantly being directed towards improving their luminous efficiency, thereby increasing the number of possible applications. To fabricate LEDs emitting near to far ultraviolet (UV) light, semiconductor layers comprising aluminum gallium nitride (AlGaN) or aluminum indium gallium nitride (AlInGaN) are typically epitaxially grown directly on a sapphire substrate using metal organic chemical vapor deposition (MOCVD). Such UV LEDs generally have an emission wavelength shorter than the energy bandgap of GaN (approximately 365 nm at room temperature). Therefore, a semiconductor layer of gallium nitride (GaN) is most likely not present outside the active layer(s) in the UV LED structure; otherwise, the emitted light would be absorbed by the GaN layer(s), resulting in a significant or total loss of efficiency.
However, the layer growth rate of AlGaN and AlInGaN is very slow, especially with high aluminum (Al) content, due to trimethylaluminum (Al(CH3)3)-ammonia (TMA-NH3) adduct formation. In addition, a thick AlGaN or AlInGaN layer will often crack, thereby limiting the thickness of the semiconductor layers in the LED stack. The stress experienced by an AlInGaN or AlGaN layer is proportional to the Al content in the layer, so the higher the Al content, the more likely it is for the AlGaN or AlInGaN layer of a given thickness to crack. For example, an AlGaN layer directly grown on sapphire with 20% Al and a thickness of 0.4 μm has been observed to develop very poor morphology with microcracking along certain preferential crystallographic directions.
Moreover, the dislocation density for AlInGaN or AlGaN layers grown directly on a sapphire substrate may be unacceptable for a desired level of performance from a UV LED. The dislocation density is a measure of how many lattice imperfections (due to the lattice mismatch between sapphire and AlGaN or AlInGaN) are present in a crystal structure of a particular volume. Since a dislocation is a line, loop, or point defect, the dislocation density is defined as the total number of defects or imperfections due to lattice mismatch per unit volume and may be expressed in units of number of dislocations/cm3. These lattice imperfections, or dislocations, may have a profound limiting effect on light emission efficiency from the LED.
Accordingly, what is needed are improved techniques to fabricate UV LEDs.